The present invention relates to a power steering system for providing steering force and/or steering assist force by actuating a hydraulic cylinder in accordance with torque input from steering input means or device such as an automotive steering wheel.
Typically, as disclosed in JP-A 57-201767, the power steering system comprises a steering shaft mounted at the center of the steering wheel, a rack-and-pinion arranged at the lower end of the steering shaft, a hydraulic cylinder or hydraulic actuator linked to the rack of the rack-and-pinion, a reversible-type oil pump for relatively supplying hydraulic fluid or pressure to first and second hydraulic chambers defined by a piston of the hydraulic cylinder through first and second passages, and a bypass valve provided to a bypass passage arranged between the first and second passages so as to open and close the bypass passage.
The bypass valve is constructed such that a valve element is opened or closed in accordance with the pilot pressures of pilot passages connected to two discharge ports of the oil pump, putting the bypass passage in communication or closing-off.
When ordinary rightward or leftward steering operation is carried out with the steering wheel during vehicle cruising, a detecting mechanism such as a steering torque sensor detects resultant steering torque to rotate the oil pump in the normal or reverse direction through a control circuit, producing the pilot pressure. Then, the bypass valve is actuated to close off communication of the bypass passage, so that the hydraulic pressure is relatively supplied from the oil pump to the hydraulic chambers, providing steering assist force to steering operation.
On the other hand, when the detecting mechanism detects no steering torque during straight-ahead cruising at medium or high velocity and the like, operation of the oil pump is stopped to allow the bypass valve to provide communication of the bypass passage. Then, hydraulic fluid can flow between the hydraulic chambers, achieving steering operation by steering force of the steering wheel only.
With the typical power steering system, however, the hydraulic circuit forms a normally-closed circuit wherein the bypass valve selectively introduces both the pilot pressure on the discharge side of the oil pump and the pilot pressure on the suction side thereof, opening or closing the bypass passage in accordance with a differential pressure therebetween.
As a result, when the steering wheel is rotated in one direction from the neutral position, the bypass valve is close off to make the hydraulic pressure act on the hydraulic chamber on the one-direction side. Then, when rotation of the steering wheel is stopped, the oil pump generally comes to a stop due to no road load acting the wheels. Although reaction force of spring systems of the wheels and the like acts in the direction of returning the steering wheel to the neutral position, the oil pump has rolling friction produced, providing the hydraulic pressure. Thus, the bypass valve is continuously closed off, leading to degraded operating feel of the steering wheel when returning it to the neutral position.